When an elevator having a plurality of ropes for supporting a load is initially installed, or when ropes are exchanged, it may be difficult to precisely match the lengths of the ropes due to a variety of factors, such as the rigidity of the wire ropes and the misalignment of equipment providing tension to the ropes. Other factors causing a difference in length among the ropes may include the differential expansion rate of the ropes, a fault of a sheave material or rope material, an eccentric load applied to an elevator car, and/or combinations thereof. These factors, and others, may result in length differences between the ropes, which can have negative consequences. For instance, if a length difference exists between the ropes of an elevator system, the ropes may be subject to uneven tension because the load is unevenly applied to the ropes. Due to a variation in length among ropes, the ropes having relatively short lengths when compared to the others may be subject to over-tension such that the wires of those ropes are often rapidly worn. In addition, the ropes having relatively short lengths may be easily deformed or broken while causing early wear of sheave grooves and other components. Furthermore, an unbalanced load between the ropes may generate vibrations in longitudinal and transverse directions, which may be directly transferred to the elevator car, making passengers feel uneasy. The above-described situation may be similar to a situation involving a vehicle having an inferior wheel alignment, which can shorten the life span of related components including tires and can deteriorate steering performance and riding comfort.
The load of an elevator car may be measured with a load cell associated with a load weighing hitch plate or by associating a plurality of load cells with a plurality of suspension ropes to determine the cumulative load of the elevator car. The hitch plate system may be supported by a support frame suspended by traction cables or ropes. The hitch plate and load cell may often be coupled directly to the elevator car and connect the car to an upper crossbeam or yoke operatively connected with the hoist or traction cables or ropes. In this manner, the load of the car may be measured at a single point at the center of the elevator car.
Other load measuring systems may incorporate a load measurement device located at the dead end hitch. In such systems, the tension member terminations is mounted to a bracket, which is in turn mounted to a plate. The plate is attached to a guiderail to fix the tension members relative to the hoistway. An edge flange is attached to the plate opposite the guiderails and a strain gauge is attached to the flange. The load exerted by the car suspended by the tension members is transmitted by the plate to the edge flange which is designed such that the force applied to the edge plate by the hoisting ropes causes a large deformation in the edge flange. The strain in the edge flange may be measured by the strain gauge. In other systems, a load weighting device may be used (with a set of springs) to determine the car weight by measuring the compression of the springs.
An alternative configuration for monitoring the load of an elevator car may utilize multiple tension members associated with multiple load cells. A load weighing device for an elevator may be located at the termination of each tension member for suspending the elevator car. A typical system includes an elevator car and counterweight suspended by a tension member within a hoistway. Terminations are fixed to the end of the tension member, which are in turn attached to a structure such as a mounting plate or beam that is fixed relative to the hoistway. A load cell is fixed between a spring and a mounting plate such that the load cell measures the weight borne by the tension member. For elevators having multiple tension members, there may be a load cell for each tension member. The total load of the elevator car is then measured by adding each of the loads measured at each of the plurality of tension members.
Thus, having uneven tension between tension members of an elevator system may not only reduce the service life of the tension members and affect the quality of a passenger's ride, but also it may even jeopardize the safety of the lift operation. Therefore, it may be advantageous to provide an elevator load measurement system for an elevator system having a plurality of tension members that compensates for the differences in rope length of the tension members and accurately measures the load of an elevator car with a single load cell. It may also be advantageous to provide such an elevator load measurement system that works in real time. Furthermore, it may be advantageous to provide an elevator load measurement system that dampens any vibration energy in the tension members.